1. Field of the Invention
This invention relates to a method for improving the stability of solutions which comprise lithium perchlorate and at least one tetraalkylammonium perchlorate salt dissolved in liquid sulfur dioxide. More particularly, it relates to a method for stabilizing such materials which involves the addition of minor amounts of a stabilizer.
2. Description of the Prior Art
A substantial amount of interest has recently been centered on the development of ambient temperature, high energy density, electrochemical cells which are light in weight and capable of providing a higher voltage than conventional cells such as nickel-camium and lead-acid systems or alkaline cells having zinc anodes. The high energy density cell systems which are currently of interest typically involve the use of active metals (metals above hydrogen in the electromotive series of elements which are unstable in an aqueous environment) as anodes in combination with nonaqueous electrolytes. As used herein, "nonaqueous" is intended to mean substantially free of water. Lithium has been of particular interest as an active metal for such high energy density cells since it is the most active of the metals in the electromotive series and has the ability in an electrochemical cell to provide the highest performance in watt-hours per kilogram of all known active metals.
In conventional electrochemical cells, cathode depolarizers are used in a form which will permit an intimate and maximum contact with an external electrical circuit, such as a set of wires connecting the electrodes of a cell, while also effecting a physical separation of the cathode depolarizer from the anode. In such cells, the cathode depolarizer is generally an insoluble, finely divided solid which is either admixed with or used as a coating over an inert conducting material, such as nickel, graphite or carbon rod, which serves as a current collector or cathode. The physical separation of the cathode depolarizer from the anode is necessary to prevent a direct chemical reaction between the anode material and the cathode depolarizer which would result in self-discharge of the cell.
Until recently, it was generally believed that a direct physical contact between the cathode depolarizer and the anode could not be permitted within an electrochemical cell. It has been discovered, however, that certain cathode depolarizers do not react chemically to any appreciable extent with active metal anodes at the interface between the anode and the cathode depolarizer. Accordingly, with materials of this type, it is possible to construct an electrochemical cell wherein an active metal anode is in direct contact with the cathode depolarizer. For example, U.S. Pat. No. 3,567,515 issued to Maricle et al. on Mar. 2, 1971, discloses the use of sulfur dioxide as a cathode depolarizer in such a cell.
Electrolytes comprised of a solution of lithium perchlorate and one or more tetraalkylammonium perchlorate salts in liquid sulfur dioxide are highly satisfactory for use in rechargeable lithium-sulfur dioxide cells. We have found, however, that these solutions are unstable at high lithium perchlorate and tetraalkylammonium perchlorate salt concentrations. For example, a one molal solution of tetraalkylammonium perchlorate in liquid sulfur dioxide which is saturated with lithium perchlorate typically begins to decompose about one hour after preparation. This decomposition is observed as a yellowing of the solution and the gradual separation of a precipitate. Unfortunately, this decomposition limits the utility of such electrolytes in lithium-sulfur dioxide cells.
The above-mentioned U.S. Pat. No. 3,567,515 (Maricle et al.) discloses an electrochemical cell comprising a lithium anode, a cathode, and a nonaqueous conductive liquid electrolyte which comprises sulfur dioxide. This patent also discloses that the electrolyte may contain a plethora of electrolyte salts which include lithium perchlorate, lithium halides, tetra(loweralkyl)ammonium salts of halogens, and tetraalkylammonium perchlorates. Further, the patent also discloses that mixtures of two or more electrolyte salts can be used. However, the patent fails to disclose that certain solutions which comprise lithium perchlorate and one or more tetraalkylammonium perchlorate salts dissolved in liquid sulfur dioxide are unstable. Further, the patent fails to either suggest or disclose any method to increase the stability of such solutions.